Thought I'd chime in as a Finn who has seen quite a lot of Kontiola. He's a very skilled playmaker and a good skater too. Based on talent alone he could be - like others have already said - the best center on the Leafs. I think his linemate at this year's worlds, soon-to-be-Blue Jori Lehter?, managed to miss almost 10 empty nets from Kontiola's passes. There are reasons why his previous visit to the NHL wasn't successful though. First of all he is often trying to make the fancy play at any cost, leading to bad giveaways (this was especially evident at this year's World Champs where Kontiola almost single-handedly gave the go-ahead goal to Canada in the quarter final - fortunately for us Finns Scrivens gave two back and we won

). He has a decent shot but his tendency to go for the pretty play leads to him not using it enough. Also, typically for a skilled player he is not overly physical though he doesn't shy away from puck battles in the corners.
In a nutshell he should definitely be tried on the PP where he truly shines. During even-strength play he'd be best paired with defensively responsible wingers who are good at finding their way close to the net - I'm thinking Komarov and Santorelli would be good linemates for him. For a Finn it's gonna be interesting to see two countrymen on the Leafs for the first time since Niklas Hagman and Vesa Toskala five years ago.